How to use the diff checker
Put the original version in the left box and the changed version in the right, then press Compare. The tool aligns the two texts line by line and shows a combined view: green lines were added in the new version, red lines were removed from the old one, and dimmed lines are unchanged and included so you can see where each change sits. A short summary tells you how many lines were added and removed. Everything runs in your browser, so even large documents compare instantly and privately.
How the comparison works
The checker uses a longest-common-subsequence algorithm — the same core idea behind the "diff" command programmers have relied on for decades. Rather than comparing line one to line one and line two to line two, it finds the longest sequence of lines that appears in both versions in the same order, treats those as the unchanged backbone, and marks everything else as an insertion or a deletion. This is what lets it handle a line inserted near the top without wrongly reporting every line below it as changed. The result is a minimal, readable set of differences that reflects what a human would consider the actual edit.
What it is good for
Writers and editors use it to compare two drafts and see precisely what a round of revisions changed. Developers use it to eyeball differences between two versions of a config file, a data export, or a snippet of code when a full version-control diff is overkill. Support and operations teams compare a working configuration against a broken one to find the line that drifted. Students and researchers compare quotations against a source to catch altered wording. Anyone who has asked "what changed between these two?" and been unable to spot it by eye is the audience for this tool.
Tips for cleaner diffs
The comparison is line-based, so it works best when each meaningful unit sits on its own line — for prose, that often means one sentence per line. If two texts differ only in spacing or trailing whitespace, run each through the "remove extra spaces" cleaner first so the diff shows genuine content changes rather than invisible formatting noise. When comparing lists, sort both sides the same way beforehand if order does not matter, so that reordering does not masquerade as additions and deletions. And remember the highlighting is directional: green means "present in the new version", red means "was in the old version", so reading the two colours together tells you exactly how to get from the first text to the second.